The present invention relates to a time slot management method for use in a time division multiple access network which includes a main station coupled to each of a plurality of substations via the cascade connection of a common transmission link and an individual user link, the management method including broadcasting substation identities in downstream information packets from the main station to the plurality of substations to allow each one of the plurality of substations upon detection of its own substation identity to transfer a predetermined amount of upstream information packets in predetermined upstream timeslots to the main station. The invention also relates to a main station and a substation realizing such a method.
Such a time slot management method, main station and substation are already known in the art, e.g. from the published European patent application with publication number 0 544 975 A1. Therein, a time slot management system is described which includes a time division multiple access system with a main station connected to a plurality of substations by means of a tree like network. Substation identities are broadcasted in downstream information packets from the main station to the substations in order to thereby allow each one of the substations upon detection of its own substation identity to transfer upstream information in upstream information packets in predefined upstream timeslots. This time slot management system allocates the time slots in a flexible and dynamically way. The needed bandwidth to transmit upstream information is requested by the substations to the main station and the substations are informed by the main station of the allocated timeslots.
A feature of the described time slot management system is that the downstream information packets are including a predetermined number of blocks each of which include a header part and an information part. In order to reduce the overhead of the downstream information packets a first step is realized to distribute the stream of substation identities over the header parts of this predetermined number of blocks and a second step is realized to performe only one error check for this predetermined number of blocks and to distribute the error check information bits over the header parts of the predetermined number of blocks. In this way, the overhead of one information packet is a reduced number of bits.
It has to be remarked that in order to keep a maximum transfer capacity between the main station and the substations it is desirable to keep the number of bits of the overhead of the information packets as low as possible.
An object of the present invention is to provide a time slot management method of the above known type but wherein the overhead of one information packet is further reduced.
According to the invention, this object is achieved due to the fact that the time slot management method of the invention includes embedding at least part of the substation identities in a physical layer operation and maintenance part. Such a physical layer operation and maintenance part is a predefined part of one of the downstream information packets which is anyway broadcasted from the main station to the plurality of substations in order to transport operation and maintenance information packets related to operation and maintenance functions.
Indeed, by embedding substation identities in unused fields of physical layer operation and maintenance parts, which are a predefined part of one of the downstream information packets and which are broadcasted anyway on a predefined regular base the number of bits of the overhead of one information packet is a reduced number of bits. This method is a time slot management method for use in a time division multiple access network which includes a main station coupled to each of a plurality of substations via the cascade connection of a common transmission link and an individual user link, the management method including broadcasting substation identities in downstream information packets from the main station to the plurality of substations to allow each one of the plurality of substations upon detection of its own substation identity to transfer a predetermined amount of upstream information packets in predetermined upstream timeslots to the main station, wherein the time slot management method additionally includes embedding at least part of the substation identities in a physical layer operation and maintenance part, the physical layer operation and maintenance part being a predefined part of one of the downstream information packets broadcasted from the main station to the plurality of substations to transport operation and maintenance information packets related to operation and maintenance functions.
In addition, the present invention is directed to a main station for inclusion in a time division multiple access network wherein the main station is to be coupled to each of a plurality of substations via the cascade connection of a common transmission link and an individual user link, the main station including a packet formatting module to insert substation identities in downstream information packets in order to broadcast the information packets from the main station to the plurality of substations to thereby allow each one of the plurality of substations upon detection of its own substation identity to transfer a predetermined amount of upstream information packets in predetermined upstream timeslots to the main station, wherein the main station includes inserting means to insert at least part of the substation identities in a physical layer operation and maintenance cell, the physical layers in one of the downstream information packets broadcasted from the main station to the plurality of substations to transport operation and maintenance information packets related to operation and maintenance functions.
Furthermore, the present invention is directed to a substation for inclusion in a time division multiple access network, the time division multiple access network including a main station being coupled to each of a plurality of substations including the substation via the cascade connection of a common transmission link and an individual user link, substation identities of the plurality of substations being broadcasted in downstream information packets from the main station to the substations in order to allow each one of the plurality of substations upon detection of its own substation identity to transfer a predetermined amount of upstream information packets in predetermined upstream timeslots to the main station, wherein the substation includes detecting means to detect in a physical layer operation and maintenance cell its own substation identity inserted by the main station in the physical layer operation and maintenance cell, the physical layer operation and maintenance cell being a predefined part included in one of the downstream information packets broadcasted from the main station to the plurality of substations to transport operation and maintenance information packets related to operation and maintenance functions.
It has to be remarked that operation and maintenance functions for e.g. an Asynchronous Transfer Mode layered model are described in e.g. the book Asynchronous Transfer Mode: Solution for Broadband ISDN written by Martin de Prycker, and more particular in Chapter 3: Description of ATM according to CCITT from page 97 to page 124, published in 1991 by Ellis Horwood Limited, ISBN 0-13-053513-3.
Also the scope of the Recommandation ITU-TI.610, March /1993, previously CCITT Recommandation described in paragraph 1.2, page 1 thereof, is to identify the minimum set of functions required to operate and maintain the Physical Layer and Asyncronous Transfer Mode ATM Layer aspects of the Broadband Integrated Services Digital Network B-ISDN user network interface. Five phases are considered in specifying the operation and maintenance OAM functions of the Broadbandxe2x80x94Integrated Services Digital Network B-ISDN which are described in more detail on page 1 and 2 of this ITU Recommandation and which are listed hereafter:
Performance monitoring;
Defect and failure detection;
System protection;
Failure or performance information;
Fault localization.
Common operation and maintenance OAM cell fields for point-to-point conncetions are described in paragraph 7, ATM Layer OAM Cell Format, pages 17-18 of this ITU-TI.610 Recommandation and the specific fields for each type of OAM cells are described in paragraph 7.2 Specific Fields for Fault Management Cell, pages 78 to 21 of this recommendation. As it can be seen from the description of these fields the OAM cell indeed includes unused Operation and Maintenance cell information fields. By inserting, following the invention, in a format similar to such above format but for point-to-multi-point connections, at least part of the substation identities in unused fields of the operation and maintenance cells less substation identities i.e. less bits are inserted in the overhead of the information packets.
An important advantage of the time slot management method following the invention is that in the event when all substation identities are inserted in physical layer and operation and maintenance parts the time slot management method is more flexible towards an increase of the number of substations. Indeed, with an increase of the number of substations more substation identities are required whereby the number of bits of a substation identity also increases. In the event when e.g. substation identities are distributed over the overhead of downstream information packets, an increase of the word length of the substation identities has an impact on the overhead of the information packets and eventually on the structure of the downstream frame format. However, in the event when all substation identities are inserted in physical layer and operation and maintenance parts, an increase of the word length of the substation identities has only an impact on the organization of the fields of the physical layer and operation and maintenance parts.
It has to be remarked that the downstream information packets and the upstream information packets may be carried as e.g. a continuous stream of cells in a cell based format with a cell based physical layer. Physical layer operation and maintenance cells are used for the conveyance of the physical layer operation and maintenance information and are inserted in the continuous stream of information packets with a predefined insertion rate. Embedding at least part of the substation identities in such physical layer operation and maintenance cells, shortly PLOAM cells, is a possible implementation of the method of the invention.
It has to be remarked that a substation identity is not necessarily a manufacturing number assigned during manufacturing following a predefined unique and programmed manufacturing serial number. It can also be a ranging grant which is used during a ranging process whereby this ranging grant is generated by a main station in order to initiate the ranging process. Two conditions for a substation to react on such a ranging grant is that the substation has not yet received an identification number from the main station and that its manufacturing serial number matches a mask given by the central station. If both conditions are valid the substation is allowed to react on such a ranging grant. A feature of the method according to the present invention is that a substation identity embedded in a physical layer operation and maintenance cell is a ranging grant.
Once a substation has received an identification number from the main station and the ranging process is completed, the substation is allowed to react on a data grant from the main station which includes his identification number and is allowed to send data to the main station.
A time slot management method following the invention can be used in an optical communication network.
Yet an important advantage of the management method following the invention becomes clear in the event when a downstream information packet is an asynchronous transfer mode cell, i.e. an ATM cell, and in the event when all substation identities are embedded in physical layer operation and maintenance cells and no total overhead is required. Indeed, for such a format the downstream frame format of the downstream information packets lines up with the requirements of the International Telecommunication Union ITU-T Recommendation I.432, March /1993, Integrated Services Digital Network ISDN User Network Interfaces/Broadband Integrated services Digital Network B-ISDN User Network Interfacexe2x80x94Physical Layer Specification. This standard describes more in detail on page 8 the interface structure of the physical layer for a cell based interface consisting of a continuous stream of ATM cells, each containing 53 octets and with a maximum spacing between successive physical layer cells of 26 ATM layer cells. Such a physical layer cell can either be an idle cell or a physical layer operation and maintenance cell depending on the operation and maintenance requirements. A time slot management method following the invention can have at least one downstream information organized as an asynchronous transfer mode cell ATM.